The present invention relates generally to a device for load dependent control and/or limitation of the braking forces of hydraulic of pneumatic vehicle brakes, and more particularly, to such brake control systems on the rear wheels of road-going utility vehicles.
Because of legal requirements and regulations, it is necessary to vary the rear wheel braking forces on utility vehicles as a function of the vehicle load and to efficiently utilize the varying adhesion conditions between the tires and the road surface. It is necessary to avoid, as far as possible, undesirable overbraking and underbraking of the rear wheels relative to the front wheels. If the rear wheels are braked too strongly (overbraking), they may lock prematurely and cause a tail slide to occur. If the rear wheels are braked too softly (underbraking), the front wheels can lock prematurely and cause the vehicle to become unsteerable.
One such device directed to this problem is known from German Unexamined Published Application (DE-OS) No. 23 40 916. In this arrangement, the valve affected by inertial forces is held open by a spring during moderate deceleration so that the additional active surface located directly on the control piston is substantially subject to the pressure applied on the input side of the control valve. With this connection, possible throttling losses can be neglected. During stronger deceleration of the vehicle, the control valve is closed due to the force of a heavy body arranged to be movable in the longitudinal direction of the vehicle. Since the valve is so arranged that its closing pressure is increased with increasing input side pressure, the pressure acting on the additional active surface is, therefore, locked in during the closing of the valve. Hence, this pressure remains substantially constant as long as the valve remains in the closed condition.
Although the threshold value of valve closing is changed as a function of vehicle load in this device, the proportionality between the pressure existing at the output side of the control valve and the pressure existing at the input side of the control valve cannot be altered. Diagramatically, this relationship of the pressure existing at the output side of the control valve (usually the pressure existing at the rear wheel brakes) as a function of the pressure existing at the input side of the control valve, for a plurality of different vehicle load conditions, can be shown as a family of characteristic lines. Typically, each such line consists of a more or less long first section having a large slope starting from the origin and is continued by a second section with a smaller slope. The first sections all have the same slope with respect to one another, and likewise the second sections all have the same slope with respect to one another.
This is not an optimum arrangement to the extent that it only gives an extraordinarily incomplete approximation of the shape of an ideal characteristic diagram. Such an ideal diagram consists of a family of hyperbola having the same slope at the origin. It follows that frequent overbraking or underbraking of the rear wheels is to be expected whenever the loading condition of the vehicle deviates from a value which is optimum in terms of braking characteristics of a particular control system.
In this respect, the devices known from German Unexamined Published Applications (DE-OS) Nos. 25 22 929 and 30 19 630 also fail to provide an ideal solution. The design of these devices is similar to that shown in German Unexamined Published Application (DE-OS) No. 23 40 916, mentioned above. The primary difference between these latter two designs and the above-mentioned design consists substantially in the fact that the additional active surfaces are located on separate pistons which are connected to the control pistons of the control valves by means of spring arrangements. These spring arrangements are clamped between the additional pistons and the control pistons. The hydraulic or pneumatic pressure acting on the additional active surfaces operates indirectly, therefore, rather than directly on the control pistons. The additional pistons are also being displaced, and the spring arrangements are subjected to an alteration in tension. The principles of operation do, however, correspond to that of the device shown in German Unexamined Published Application (DE-OS) No. 23 40 916.
A pressure control device for a vehicle brake installation is also shown in German Unexamined Published Application (DE-OS) No. 25 02 265. In this device, the control piston can be subjected to an additional force by means of an additional piston arrangement and an inertia pendulum reacting to deceleration of the vehicle. This additional force is dependent upon the vehicle deceleration and the difference between the pressure existing between the input side of the control valve and the pressure existing at the output side of the control valve. However, the inertia pendulum is located as a force transmission element between the control piston and the additional piston arrangement. Thus, the inertia pendulum itself reacts to the differential pressure so that oscillations of the pendulum can have an undesirable influence on the operation of the control valve. Further, the inertia of pendulum has other adverse influences on the operation of the control valve.
It is therefore an object of the present invention to provide a brake control system which prevents undesirable overbraking and underbraking of the rear wheels relative to the front wheels.
Another object is the provision of an improved brake control system wherein a change in the proportionality between the increase in the output side pressure and the increase in the input side pressure as a function of the vehicle load can be attained with good reproducibility in a simple constructional manner.
These and other objects of the present invention are attained in the provision of a control valve upstream of the brakes and having therein a control piston arranged as a valve body. This control piston is subjected to hydraulic or pneumatic pressure applied to the input side of the control valve and/or to mechanical preloading in the opening direction and to hydraulic or pneumatic pressure applied to the output side of the control valve in the closing direction. Mechanical preloading can be provided by a spring means. The output side of the control valve may lead directly to the brake means. In this way, the control valve limits the output side pressure relative to the input side pressure above a load dependent threshold value of the input side pressure. To achieve load dependent control of the threshold value at the control piston or at a part with a drive connection to the control piston, an additional active surface is subject to pneumatic or hydraulic pressure via a pneumatic or hydraulic connection branching off from the input pressure side line. A valve is located in this connection which can be effected by inertia forces during deceleration of the vehicle in order to control the production of forces acting to open the control valve and permit pressure flow to the brakes. This valve arrangement which can be influenced by inertia forces is generally referred to herein as a seat-controlled valve and reduces the pressure to the additional active surface on the output side of this valve arrangement as a function of the vehicle deceleration dependent closing pressure of the seat-controlled valve relative to the input side pressure.
Therefore, the seat-controlled valve can be opened under the influence of the input side pressure if the input side pressure exceeds the pressure acting on the additional active surface by a sufficiently large amount, i.e., if the input side pressure exceeds the pressure acting on the additional active surface at least by the amount of the closing pressure of the seat-controlled valve. This closing pressure is a function of the vehicle deceleration forces. The pressure acting on the additional active surface increases correspondingly with increasing input side pressure. The proportionality factor between the input side pressure and the pressure acting on the additional active surface depends on the vehicle loading because of the relationship between the vehicle deceleration and the input side pressure. Therefore, the relationship between the vehicle deceleration and the closing pressure of the seat-controlled valve depends on the vehicle loading.
Diagramatically, the present invention permits a much better approximation to the theoretical optimum relationship between the output side pressure and the input side pressure at various loading conditions.
The vehicle deceleration dependent closing pressure of the seat-controlled valve is attained, in the simplest case, in a valve body which is designed as a heavy body movable in a longitudinal direction of the vehicle between open and closed positions. This permits different inertial forces occurring at different decelerations to be utilized to produce a change in the closing pressure of the seat-controlled valve proportional to the vehicle deceleration.
In an alternate embodiment of the present invention, the seat-controlled valve can also have a light valve body which can be directly or indirectly loaded in the closing direction by a heavier body which is movably located in the vehicle longitudinal direction. This construction provides the advantageous possibility of including a spring element between the light valve body and the heavy body so that during opening and closing movements of the valve body only small inertia forces arise, due to the low mass of the valve body. Thus, the heavy body acts only as a load on the light valve body, but has no influence on the inertia of that valve body. By this means, opening and closing movements are possible in extremely rapid succession, if so required in a given brake control system.
The vehicle deceleration dependent closing forces can also be produced by an electrical control. For example, an electromagnetic coil may be employed. Such an arrangement is particularly advantageous when an actual value sensor is already provided on the vehicle, for example, in anti-brake locking systems, which records the changes in the rotational speed of the wheels and generates a deceleration-dependent signal which can be used for controlling the closing forces.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, embodiments in accordance with the present invention.